There is already really good music, but in cases some cases I feel that there aren't the "music" or "sound" I want to get a certain feeling on the map. Is it possible to add .ogg files into a mission/level? And how?

It should be fairly easy, at least for music. All the musics are in Data\Music , and the game chooses the correct music based on the playlists in the Mission folder. So for example in Shadowgrounds Survivor, the fifth Marine mission (Oops), the playlist is located in Data\Missions\marine05_oops\playlists\action.txt . There's also a separate action playlist called calm.txt (see what we did there?). The action playlist is triggered by scripts (I think there's something like musicPlaylist or so, and at least startMusic, stopMusic and nextMusic).

Ambient music/sounds are located in Mission\replace\ambient\ambient.txt , and they are triggered by the player's position in a level.